Just for your interest the Fuji XE1 does ISO6400 pretty well. Although I think Fuji overstates their ISO a bit.

I originally looked at the X-Pro 1, which uses the same sensor but I suspect that Fuji uses software NR even in RAW so there is less play in the final product . . . and Fuji also appears to be one stop less sensitive at any given ISO. But they do have some nice lenses.